37. Sudoku Solver leetcode python new season 2016

Write a program to solve a Sudoku puzzle by filling the empty cells.

Empty cells are indicated by the character '.'.

You may assume that there will be only one unique solution.

A sudoku puzzle...

class Solution(object):    def valid(self, x, y, board):        tmp = board[x][y]        board[x][y] = '.'        for row in range(9):            if board[row][y] == tmp:                return False        for col in range(9):            if board[x][col] == tmp:                return False        for row in range(3):            for col in range(3):                if board[(x / 3) * 3 + row][(y / 3) * 3 + col] == tmp:                    return False        board[x][y] = tmp        return True    def dfs(self, board):        for row in range(9):            for col in range(9):                if board[row][col] == '.':                    for char in '123456789':                        board[row][col] = char                        if self.valid(row, col, board) and self.dfs(board):                            return True                        board[row][col] = '.'                    return False        return True    def solveSudoku(self, board):        """        :type board: List[List[str]]        :rtype: void Do not return anything, modify board in-place instead.        """        self.dfs(board)